1. Field of the Invention
This invention relates to a method of selecting receiving frequency and, more particularly, to a method of selecting a receiving frequency for a radio data system receiver.
2. Description of the Prior Art
As a well known radio service, a radio data system (hereinafter referred to as RDS) is common in Europe for providing a radio service for broadcast wave listeners. In such RDS, informative data relating to broadcast programs are transmitted simultaneously with the broadcast programs in a multiplex modulation from broadcasting stations and, upon receiving the broadcast waves, a desired broadcast program is selected by the broadcast wave listener based on demodulated data.
The RDS is a data system standardized in the Europe Broadcasting Union (EBU), wherein the data relating to the broadcast such as broadcast programs and the like are coded into a two phase Differential Phase Shift Keying (DPSK) signal having a bit rate of 1187.5 bps, and these coded data are broadcasted by frequency-modulating a sub-carrier in such that modulating a 57 KHz sub-carrier in accordance with a double-sideband carrier suppression amplitude modulation.
In the RDS, as shown in FIG. 1, the whole data are transmitted as a unit which is called a group consisting of 104 bits. One group in turn consists of four blocks and each of which consists of 26 bits. Each block consists of 26 bits wherein 16-bit information is coded into an abbreviated cyclic code and then 10-bit offset word that corresponds to the same portion within the four blocks is added thereto. This offset word provides synchronization among the group structure at the time of receiving the data. The data contained in each group are prescribed depending on their locations, whereby 16 bits of the first block always designate a program identification code (PI code), while in the second block, first 5 bits designate a group-type code, the next one bit designates a traffic-program identification code (TP code), and the next succeeding 5 bits designate a program type code (PTY code).
Further, the contents of the remaining bits in the second block as well as data contained in the third and fourth blocks are determined respectively for every type of group. The group-type can be distinguished from each other by 5-bit information in total, in which the first 4 bits can provide 16 different types of 0-15 and, then, the remaining one bit provides two versions of A and B for the 16 different types of 0-15, respectively.
For example, in an OA group shown in FIG. 2, a traffic announcement identification code (TA code) is allocated in the second block, a list of alternative frequencies data (AF data) for identifying broadcasting network stations which broadcast the same program are allocated in the third block, and that program service name data (PS data) for providing a broadcast information service such as names of broadcasting stations, names of broadcasting networks and the like in the fourth block.
In a motor vehicle-mounted RDS receiver, quite often poor reception is encountered while listening to broadcast wave during a motor vehicle travel. However, in the RDS broadcast, since the AF data of the broadcasting stations within the same broadcasting network, which stations are broadcasting the same broadcast program on different frequencies, are available as described above, it is possible to select another broadcasting station in the same broadcasting network having a stronger electric field strength, or a greater signal level, than that of the broadcasting station presently tuned in based on the AF data stored in a memory for chasing the same broadcast program.
In FIG. 3, for example, broadcasting stations A, B, C, D and E form a group of broadcasting stations that belongs to the same broadcasting network. Within the same broadcasting network, all broadcasting stations broadcast the same broadcast program but on different frequencies based on corresponding AF data f.sub.A, f.sub.B, f.sub.C, f.sub.D and f.sub.E, respectively. It is assumed that each broadcasting station transmits the AF data, which represent frequencies data of neighboring broadcasting stations, in such a way as shown in the following table 1.
TABLE 1 ______________________________________ Station Station Station Station Station A B C D E ______________________________________ AF DATA f.sub.B f.sub.A f.sub.A f.sub.C f.sub.B f.sub.C f.sub.C f.sub.B f.sub.E f.sub.D f.sub.Z f.sub.D f.sub.D f.sub.V f.sub.V f.sub.X f.sub.E f.sub.X f.sub.W f.sub.W f.sub.Z f.sub.B f.sub..sub.Y ______________________________________
Firstly, when the motor vehicle driver selects the broadcasting station A by operating a tuner of the vehicle-mounted RDS receiver for receiving a desired broadcast program. Secondly, when the motor vehicle leaves a service area of the broadcasting station A and enters the service area of the broadcasting station B. It is possible to switch the receiving frequency of the vehicle-mounted RDS receiver from that of the broadcasting station A to B based on the AF data (f.sub.B, f.sub.C, f.sub.Z, f.sub.X) transmitted by the broadcasting station A since the AF data f.sub.B designates that the same broadcast program is available on a corresponding broadcast frequency of the broadcasting station B. Similarly, if the motor vehicle travels along, for example, a route (via the broadcasting stations A-B-C-D-E) passing through the respective broadcasting service areas as shown in FIG. 3, the vehicle-mounted RDS receiver can receive broadcast waves successively, after leaving the service area of the broadcasting station A, via the broadcasting stations B, C, D and, then finally, E, thus providing the motor vehicle-mounted RDS receiver with good reception of the desired broadcast program.
The receiving frequency selecting method of the prior-art-type RDS receiver has been described hereinabove. In such an RDS receiver, after the reception of the broadcast wave is switched over in succession from the broadcasting station A to the broadcasting station E via the broadcasting stations B, C and D, if the motor vehicle travels around a mountainous district along a route, shown by a dotted line in FIG. 3, and returns to the service area of the broadcasting station A, whereas the motor vehicle once has left the broadcasting service area of the network and returned to the service area of the broadcasting station A again, the reacquisition of the same broadcast program becomes very hard to maintain unless the AF data for the broadcasting station A is included in the AF data group transmitted by the broadcasting station E.
It is therefore an object of this invention to eliminate the problems encountered by the prior-art-type RDS receiver and to provide a new receiving frequency selecting method for a RDS receiver wherein it is assured for a motor vehicle driver to resume the same broadcast program in case that the motor vehicle, after it once has left a service area of a broadcasting network, returns back to the service area of the broadcasting network, within which the same broadcast program has been received from a plurality of broadcasting stations, or to a broadcasting service area neighboring thereto.